1. Field of the Invention
The present invention relates generally to an apparatus and method for providing transmit diversity in a wireless communication system, and in particular, to an apparatus and method for providing transmit diversity in a wireless communication system having multiple antennas.
2. Description of the Related Art
In common, a wireless communication system is a system developed for users to perform communication without the limitation of wired communication. The wireless communication system has been progressed from a system for providing basic voice communication to a system for providing a high-speed data service. Thus, a method of transmitting data at a higher rate has been required. Various methods of transmitting data at a high rate are used, and one of them is a method of providing transmit diversity using multiple antennas. Another one is a method of transmitting data at a high rate using multi-input multi-output (MIMO) orthogonal frequency division multiplexing (OFDM) technology. Research and development have been being continued for improving communication quality with performing higher-speed communication using these technologies in a mixed manner.
One of technology standards using the above-described methods is “IEEE P802.16-REVd/D5-2004, Part 16: Air Interface for Fixed Broadband Wireless Access Systems”, which is a wideband wireless communication standard aiming at a fixed or mobile wireless Internet service. In IEEE P802.16-REVd, a single media access control layer (MAC) and four physical layers (PHYs) are defined. Among the four PHYs of IEEE P802.16-REVd, an OFDM or OFDMA method takes attentions, and a large portion of a MIMO based system standard including space time coding (STC) is included. For the convenience of description, the contents of IEEE P802.16d are called OFDM.
A data transmission structure of the OFDM method will now be described.
FIG. 1 illustrates a frame structure suggested in IEEE 802.16d.
FIG. 1 shows a plurality of frames, an (n−1)th frame, an nth frame 100, and an (n+1)th frame. The frames have the same format. Thus, the nth frame 100 will now be described.
The nth frame 100 is composed of an nth sub-frame (n) 110 of a downlink (DL) and an nth sub-frame (n) 120 of an uplink (UL). Each of the nth sub-frame (n) 110 of the DL and an nth sub-frame (n) 120 of the UL includes a plurality of bursts. That is, the nth sub-frame (n) 110 of the DL includes a plurality of DL bursts 111, 112, 113, . . . , and the nth sub-frame (n) 120 of the UL includes a plurality of UL bursts 121, 122, 123, . . . .
Since each of the DL bursts and the UL bursts has the same structure, only the DL burst 112 and the UL burst 122 will be described. The DL burst 112 includes a preamble symbol 112a at the beginning and data symbols 112b, . . . , and the UL burst 122 includes a preamble symbol 122a at the beginning and data symbols 122b, . . . . The DL preamble symbol 112a and the UL preamble symbol 112a may have different formats.
That is, the DL preamble symbol 112a can be defined in a PEVEN or PSTC pattern, and the UL preamble symbol 112a can be defined in a PEVEN or PSUB pattern. Preamble patterns will now be described.
The patterns for preamble symbols are classified into several types according to the usage of the preamble symbols. A first preamble pattern is a PALL pattern, and a second preamble pattern is a PSUB pattern. The PALL pattern and the PSUB pattern are defined as Equation 1 and Equation 2, respectively.
                                          P            ALL                    ⁡                      (                          k              =                                                -                  100                                ⁢                                  :                                ⁢                100                                      )                          =                  [                                    1              -              j                        ,                          1              -              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              -              j                        ,                          1              -              j                        ,                                          -                1                            +              j                        ,                          1              -              j                        ,                          1              -              j                        ,                          1              -              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              -              j                        ,                                          -                1                            +              j                        ,                          1              -              j                        ,                          1              -              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              -              j                        ,                          1              -              j                        ,                                          -                1                            +              j                        ,                          1              -              j                        ,                          1              -              j                        ,                          1              -              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              -              j                        ,                                          -                1                            +              j                        ,                          1              -              j                        ,                          1              -              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              -              j                        ,                          1              -              j                        ,                                          -                1                            +              j                        ,                          1              -              j                        ,                          1              -              j                        ,                          1              -              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                                          -                1                            +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                          1              -              j                        ,                                          -                1                            +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            +              j                        ,                          1              -              j                        ,                                          -                1                            +              j                        ,                                          -                1                            +              j                        ,                          1              -              j                        ,                                          -                1                            +              j                        ,                          1              -              j                        ,                          1              -              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                                          -                1                            +              j                        ,                          1              -              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              -              j                        ,                                          -                1                            +              j                        ,                          1              -              j                        ,                          1              -              j                        ,                                          -                1                            +              j                        ,                          1              -              j                        ,                                          -                1                            +              j                        ,                                          -                1                            +              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,            0            ,                                          -                1                            -              j                        ,                          1              +              j                        ,                                          -                1                            +              j                        ,                                          -                1                            +              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              -              j                        ,                          1              -              j                        ,                          1              -              j                        ,                                          -                1                            +              j                        ,                                          -                1                            +              j                        ,                                          -                1                            +              j                        ,                                          -                1                            +              j                        ,                          1              -              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                                          -                1                            +              j                        ,                          1              -              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            +              j                        ,                          1              -              j                        ,                          1              -              j                        ,                          1              -              j                        ,                                          -                1                            +              j                        ,                          1              -              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            +              j                        ,                                          -                1                            +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                          1              -              j                        ,                          1              -              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            +              j                        ,                                          -                1                            +              j                        ,                                          -                1                            +              j                        ,                          1              -              j                        ,                          1              -              j                        ,                          1              -              j                        ,                          1              -              j                        ,                                          -                1                            +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                                          -                1                            +              j                        ,                                          -                1                            +              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              -              j                        ,                          1              -              j                        ,                          1              -              j                        ,                                          -                1                            +              j                        ,                                          -                1                            +              j                        ,                                          -                1                            +              j                        ,                                          -                1                            +              j                        ,                          1              -              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              -              j                        ,                                          -                1                            +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              -              j                        ,                                          -                1                            +              j                        ,                                          -                1                            +              j                        ,                                          -                1                            +              j                        ,                          1              -              j                        ,                                          -                1                            +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              -              j                        ,                          1              -              j                                ]                                    (        1        )            
                                          P            SUB                    ⁡                      (                          k              =                                                -                  100                                ⁢                                  :                                ⁢                100                                      )                          =                  [                                    1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                                          -                1                            +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              -              j                        ,                                          -                1                            -              j                        ,                          1              -              j                        ,                          1              +              j                        ,                          1              -              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              -              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                                          -                1                            +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              -              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              -              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              -              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                                          -                1                            +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              -              j                        ,                                          -                1                            -              j                        ,                          1              -              j                        ,                                          -                1                            -              j                        ,                                          -                1                            +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            +              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                                          -                1                            +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              -              j                        ,                                          -                1                            -              j                        ,                          1              -              j                        ,                                          -                1                            -              j                        ,                                          -                1                            +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            +              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,            0            ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                                          -                1                            +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              -              j                        ,                                          -                1                            -              j                        ,                          1              -              j                        ,                          1              +              j                        ,                          1              -              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              -              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                          1              -              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            +              j                        ,                          1              +              j                        ,                          1              -              j                        ,                                          -                1                            -              j                        ,                                          -                1                            +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            +              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                          1              -              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            +              j                        ,                          1              +              j                        ,                                          -                1                            +              j                        ,                          1              +              j                        ,                          1              -              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              -              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                                          -                1                            +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              -              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                                          -                1                            +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            +              j                        ,                                          -                1                            -              j                        ,                          1              +              j                        ,                                          -                1                            -              j                        ,                                          -                1                            -              j                                ]                                    (        2        )            
The index k in Equations 1 and 2 is an index value based on a frequency offset index (FOI). In Equations 1 and 2, when k<−100 or k>100, each of the PALL pattern and the PSUB pattern has a value of ‘0’.
A third preamble pattern is the PEVEN pattern, and a fourth preamble pattern is a PODD pattern. The PEVEN pattern and the PODD pattern are defined as Equation 3 and Equation 4, respectively.
                                          P            EVEN                    ⁡                      (            k            )                          =                  {                                                                                          2                                    ·                                      P                    ALL                                                                                                                    k                                          mod                      ⁢                      2                                                        =                  0                                                                                    0                                                                                  k                                          mod                      ⁢                      2                                                        ≠                  0                                                              }                                    (        3        )                                                      P            ODD                    ⁡                      (            k            )                          =                  {                                                    0                                                                                  k                                          mod                      ⁢                      2                                                        =                  0                                                                                                                          2                                    ·                                      P                    ALL                                                                                                                    k                                          mod                      ⁢                      2                                                        ≠                  0                                                              }                                    (        4        )            
In Equations 3 and 4, the FOI based index k=−N/2, −N/2+1, . . . , −1, 0, 1, . . . , N/2−2, and N/2−1 (N=256).
A final preamble pattern is the PSTC pattern used when two antennas are used. The PSTC pattern is defined as Equation 5.
                                          P            STC                    ⁡                      (            k            )                          =                  {                                                                                          P                    EVEN                                    ⁡                                      (                    k                    )                                                                              for                                            first                                            antenna                                            path                                                                                                          P                    ODD                                    ⁡                                      (                    k                    )                                                                              for                                            second                                            antenna                                            path                                              }                                    (        5        )            
As illustrated in Equations 1 through 5, the preamble patterns have the following characteristics. In the PEVEN pattern, a signal exists in only even sub-carriers not odd sub-carriers. In the PODD pattern, a signal exists in only odd sub-carriers not even sub-carriers. In the PSUB pattern, a signal exists in both even sub-carriers and odd sub-carriers. In the PSTC pattern, a first antenna ant0 uses the PEVEN pattern, and a second antenna ant1 uses the PODD pattern.
An OFDM burst structure available from the preambles will now be described with reference to FIG. 2.
FIG. 2 illustrates an OFDM burst structure when bursts are transmitted in an OFDM method between a transmitter and a receiver. A structure and a transmission method of OFDM bursts transmitted and received in the OFDM method between the transmitter and the receiver will now be described with reference to FIG. 2.
FIG. 2 illustrates an access point or base station 210 and a terminal 220 transmitting and receiving data in the OFDM method. A direction from the base station 210 to the terminal 220 is called downlink, and a direction from the terminal 220 to the base station 210 is called uplink. Thus, a DL is a link formed from the base station 210 to the terminal 220, and a UL is a link formed from the terminal 220 to the base station 220. In addition, FIG. 2 illustrates that each of the base station 210 and the terminal 220 has two antennas. Thus, for the base station 210 and the terminal 220, an antenna numbered as 0 denotes a first antenna, and an antenna numbered as 1 denotes a second antenna.
When a MIMO OFDM system having the above-described configuration is constructed based on the frame structure and the preamble structure suggested in IEEE P802.16d, all configurable burst structures are shown in FIG. 2. That is, a DL burst structure can be a “PEVEN pattern+data symbols” format 231 or a “PSTC pattern+data symbols” format 232. A UL burst structure can be a “PEVEN pattern+data symbols” format 241 or a “PSUB pattern+data symbols” format 242.
A preamble symbol included in a transmitted burst is used to estimate a channel in the receiver. Thus, if the receiver cannot receive the preamble symbol or perform the channel estimation from the preamble symbol, the receiver cannot correctly recover transmitted data. Thus, the preamble symbol is a very important symbol in the MIMO OFDM system.
As described above, if a preamble uses the PEVEN pattern, the PODD pattern, or the PSTC pattern in the burst structure of the MIMO OFDM system, only even or odd sub-carriers are transmitted without transmitting all sub-carriers. In this case, the receiver estimates a channel for the even or odd sub-carriers for the channel estimation. Then, the receiver must estimate a channel for the unused odd or even sub-carriers through interpolation from the channel estimated using the preamble.
In a wireless communication system, the channel estimation is very important. Thus, in general, various schemes are used to estimate a channel in the wireless communication system. One of the channel estimation schemes is a diversity scheme. The diversity scheme is divided into a receive (Rx) diversity scheme and a transmit (Tx) diversity scheme.
In the wireless communication system, a MIMO scheme is used to transmit data at a high rate. The MIMO scheme is a scheme for improving transmission efficiency by increasing the number of transmit streams using a plurality of antennas. Theoretically, if the MIMO scheme is used, since the number of transmit and receive antennas increases without an additional increase of frequency bandwidth, the serviceable data capacity increases linearly proportional to the increase of the number of transmit and receive antennas.
Besides the MIMO scheme, techniques using the diversity scheme are disclosed in various fields of the wireless communication system.
The Rx diversity scheme will now be described. When the Rx diversity scheme is used, the receiver must has a plurality of antennas in general. Thus, it is difficult to implement the receiver, and complexity of the receiver increases.
The Tx diversity scheme is divided into an open-loop scheme and a closed-loop scheme according to whether feedback information is received. In the Tx diversity scheme, data is transmitted by increasing Tx diversity by providing a specific gain to a signal, antenna, or symbol to be transmitted according to the open-loop scheme or the closed-loop scheme. Then, the receiver can more efficiently receive the transmitted data. That is, if the Tx diversity scheme is used, symbols can be more safely transmitted through a channel.
Tx diversity schemes disclosed till now will be described.
Firstly, a Tx diversity scheme of an STC scheme is suggested in U.S. Pat. No. 6,775,329 invented by Alamouti, et al. This scheme is a scheme of obtaining diversity in a space axis and a time axis using more than two antennas. The scheme suggested by Alamouti, et al. is a scheme of performing Tx diversity using a relatively simple structure and performs encoding in the time axis to have a correlation between adjacent symbols using more than two antennas. Thus, the scheme suggested by Alamouti, et al. must have a structure for gathering at least two symbols for STC and processing the gathered symbols. That is, this limitation introduces that the scheme suggested by Alamouti, et al. cannot be used unless a standard specification supports a scheme of gathering at least two symbols and processing the gathered symbols.
Secondly, a Tx diversity scheme using an “equal power joint maximum ratio combining” scheme based on a beam-forming scheme is suggested in U.S. Pat. No. 6,785,520 invented by Sugar, et al. However, the scheme suggested by Sugar, et al. has a problem of using the beam-forming scheme. That is, when the beam-forming scheme is used, a receiver must calculate a weight vector having an optimal solution, and Tx diversity must be performed by applying the weight vector to a transmission signal. In general, a calculation process for obtaining the weight vector for the beam-forming is very complicated. Furthermore, a complex calibration process is needed to obtain the Tx diversity, and a spectrum of the transmission signal must follow a standard specification.
Thirdly, a Tx diversity scheme is suggested in U.S. Pat. No. 6,763,225 invented by Farmine, et al. This scheme is a “phase alignment” Tx diversity scheme, which can be applied to a Code Division Multiple Access-Time Division Duplex (CDMA-TDD) system. The scheme suggested by Farmine, et al. is based on a scheme for a base station to receive a channel weight vector fed back from each of a plurality of mobile terminals and process the fed-back channel weight vector. Since the base station uses a phase alignment scheme between weight vectors of mobile terminals to align phases of channel weight vectors of all mobile terminals, the scheme suggested by Farmine, et al. is simpler than a conventional joint optimization scheme. However, since this scheme has a structure applied to a CDMA TDD scheme and uses channel weight vectors fed back from terminals, additional hand-shaking is necessary, and since the phase alignment is applied to weight vectors between terminals in a time domain processing method, a filter is necessary, and thereby, implementation is complicated.
Besides, a Tx diversity scheme applicable to a CDMA-TDD system is suggested in U.S. Pat. No. 6,754,286 invented by Hottinen, et al., and a Tx diversity scheme based on an OFDM system is suggested in U.S. Pat. No. 6,473,393 invented by Ariyavisitakul, et al. In addition, a scheme of optimizing a signal-to-noise ratio (SNR) through an interactive process using a time domain processing method using a “tapped delay line filter” is suggested in U.S. Pat. No. 6,873,651 invented by Tesfai, et al., and a Tx diversity scheme of optimizing an SNR using a partial channel state information (CSI) transmission scheme and a beam-forming scheme is suggested in U.S. Pat. No. 6,862,271 invented by Medvedev, et al. However, the above-described schemes require a series of hand-shaking or are still complicated.
That is, the above-described schemes are unreasonable to be applied to the OFDM scheme, or even if the above-described schemes can be applied to the OFDM scheme, terminals are complicated or a series of hand-shaking between a transmitter and a receiver is required, thereby resulting the waste of a bandwidth.